Carbon Dioxide in Equilibrium

Carbon Dioxide in the Ocean

The levels of carbon dioxide in the atmosphere has been increasing since the beginning of the industrial revolution due to increased combustion of fossil fuels. Much of the carbon dioxide that has been released dissolves in the ocean.

We can use Henry's Law to calculate the concentration of dissolved carbon dioxide in an aqueous solution. The current, average concentration of CO2 is 387 ppm, that is 387 x 10-6 atm.

[CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M

Dissociation of Carbon Dioxide

Carbon dioxide reacts with water. The carbon atom of CO2 is electron poor with an oxidation state of IV. The electron rich oxygen of water donates an electron pair to the carbon. After proton transfer from water to an oxygen of the CO2 unit, carbonic acid is formed.

The reaction between water and dissolved carbon dioxide is reversible and rapid.

Carbonic acid is in equilibrium with the bicarbonate anion.

We can combine the two equilibria:

Because the concentration of dissolved carbon dioxide is set by Henry's Law, we can use this to determine the concentration of protons and bicarbonate ion in water.

Temperature Dependence

As with other gases, the solubility of carbon dioxide in water decreases as the temperature increases. You can see this for yourself by observing what happens when you heat a can of soda.

As the temperature of the oceans increases with increasing global temperature (caused by greenhouse gases), carbon dioxide will tend to outgas from the oceans. This will increase the atmospheric concentration of CO2, causing further increases in temperature of the atmosphere, etc.